Elastography is a technique to measure the mechanical properties of soft tissue in order to detect pathological or anatomical changes. For this purpose, an imaging modality such as ultrasound or magnetic resonance imaging (MRI) is typically used to capture tissue displacements due to an externally applied excitation. The measured displacements are then analyzed to estimate the mechanical properties such as elasticity, viscosity, shear wave speed, Poisson's ratio or nonlinear tissue parameters. The quasi-static and low-frequency compression schemes, although proven to be effective in many cases, cannot provide quantitative estimates of the viscoelastic parameters without accurate knowledge of boundary forces. In order to obtain absolute values of the mechanical properties, dynamic or transient elastography solutions have been proposed in prior art. In some of these methods, such as those described in U.S. Pat. No. 7,731,661 to Salcudean et al. and in U.S. patent application Ser. No. 12/240,895 to Salcudean et al. and in U.S. patent application Ser. No. 12/611,736 to Eskandari et al. the entirety of each of which is hereby incorporated by reference, a mechanical exciter induces a dynamic motion field in soft tissue while an ultrasound machine tracks the motion of the tissue due to that exciter. In dynamic elastography, a steady-state excitation is applied to the tissue and the steady-state wave patterns at specific frequencies are observed.
A major limitation of conventional medical imaging devices, such as ultrasound and MRI, is that the image acquisition frame rate is small relative to the excitation frequency that is required for dynamic elastography purposes.
In order to address this limitation in magnetic resonance elastography, the motion sensitive gradient is generally synchronized with the harmonic excitation to sample at equally spaced time intervals. This method is described in U.S. Pat. No. 6,486,669 to Sinkus et al. the entirety of which is hereby incorporated by reference. Therefore, the imaging mechanism is phase locked to the harmonic excitation waveform and can reproduce the actual motion pattern. This method requires accurate synchronization between the imaging device (which is MRI in this case) and the excitation. Another drawback of this technique is that it only allows sampling of the waveform at particular instances within every period.
In ultrasound elastography, one can increase the imaging frame-rate by scanning a small region and repeating the procedure until the entire region of interest is scanned (as used for example in a journal paper by Nightingale, et al. “On the feasibility of remote palpation using acoustic radiation force”, in Journal of the Acoustical Society of America, vol. 110, no. 1, pp. 625-634, 2001). Alternatively, to sample high frequency tissue motion using an ultrasound machine, a different scheme for data acquisition, processing or transducer pulse sequencing can be employed. In a paper by Brekke, et al. in 2004 (“Increasing frame rate in ultrasound imaging by temporal morphing using tissue Doppler,” in Proceedings of the IEEE Ultrasonics Symposium, pp. 118-121) morphing is used to produce additional frames between successive acquired frames. The velocity data obtained from the ultrasound data is used in the morphing algorithm. Although this method helps the clinicians by increasing the display update rate, it does not overcome the inherent Nyquist limit for measuring high-frequency components of tissue motion. Another approach to high-frame-rate ultrasound imaging is to use non-conventional pulsing techniques. One of these techniques which was entitled Explososcan and discussed in a paper by Shattuck, et al. in 1984 (“Explososcan: A parallel processing technique for high speed ultrasound imaging with linear phased arrays,” Journal of Acoustical Society of America, vol. 75, no. 4, pp. 1273-1282) is based on transmitting an unfocused ultrasound pulse such as one with a linear wave front. The pulse illuminates the entire region to be imaged. Parallel receive beam-forming is then performed to reconstruct the radio frequency (RF) data and B-mode images. In another paper by J. Lu in 1997 (“2D and 3D high frame rate imaging with limited diffraction beams”, in IEEE Transactions in Ultrasonics, Ferroelectrics & Frequency Control, vol. 44, no. 4, pp. 839-856), the inverse Fourier transform was used to construct an ultrasound image from such transmitted and received plane or linear wave fronts. Fink et al. in 2002 (Ultra high speed imaging of elasticity”, in IEEE Ultrasonics Symposium, pp. 1811-1820), proposed the use of ultrafast ultrasound which is a similar technique based on unfocused transmission and parallel receive beam-forming. In a paper by Baghani et al. in 2004 (“A high frame rate ultrasound system for the study of tissue motions”, in IEEE Transactions in Ultrasonics, Ferroelectrics & Frequency Control, vol. 57, no. 7, pp. 1535-1547), the entirety of which is hereby incorporated by reference, a high frame-rate ultrasound technique based on beam-interleaving and sectoring of the original field of view is proposed. Additional phase correction was implemented to compensate for the phase difference between adjacent sectors.
The techniques developed in the past few decades are based on complicated hardware and data processing. One of the deficiencies of such methods, implemented for example on ultrasound, is that the resulting ultrasound images are of reduced quality compared to a conventional ultrasound data acquisition scheme with sequential A-line acquisition. However, a conventional ultrasound machine which can acquire sequential A-lines at a lower frame-rate cannot track tissue motion at high frequencies using any of the prior art methods. Prior art methods do not enable capturing of high frequency tissue motion with conventional medical imaging devices which is for example required for measuring the absolute values of the mechanical properties. Measurement of the absolute values of tissue mechanical properties generally requires a vibration frequency of at least 25 Hz and thus a minimum sampling rate of 50 Hz which cannot be achieved on a conventional ultrasound machine unless by lowering the quality of the images. It is beneficial to be able to measure tissue displacements with a conventional imaging device that creates images sequentially without resorting to much reduced sizes of regions of interest. Also, an MRI machine cannot acquire tissue displacements with an arbitrary waveform at any sampling frequency.
Consequently, there exists a need for a method that addresses one or more deficiencies in the prior art. The present invention enables the measurement of high-frequency tissue displacements which is otherwise impossible using a relatively low frame-rate imaging device.